As a result of the interest in recent years to reduce the emission of pollutants from large industrial furnaces employing a plurality of burners significant improvements have been made in burner design. In the past, burner design improvements were aimed primarily at improving heat distribution. Increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants and to methods to reduce emissions from to the furnace itself.
Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to, nitrogen oxide and nitrogen dioxide. Reduction of NOx emissions is a desired goal to decrease air pollution and meet government regulations.
The rate at which NOx is formed is dependent upon the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of NOx increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less NOx than the same mixture at a lower temperature, over a longer period of time.
A strategy for achieving lower NOx emission levels is to install a NOx reduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, represents a less desirable alternative to improvements in burner design.
Burners used in large industrial furnaces may use either liquid fuel or gas. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and combustion air is mixed with the fuel at the zone of combustion.
Gas fired burners can be classified as either pre-mix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.
Raw gas burners inject fuel directly into the air stream, and the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763. In addition, many raw gas burners produce luminous flames.
Pre-mix burners mix the fuel with some or all of the combustion air prior to combustion. Since pre-mixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Pre-mixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, pre-mix burners are often compatible with various steam cracking furnace configurations.
Floor-fired pre-mix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a pre-mix burner is the burner of choice for such furnaces. Pre-mix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
One technique for reducing NOx that has become widely accepted in industry is known as combustion staging. With combustion staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NOx emissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase as well.
The majority of recent low NOx burners for gas-fired industrial furnaces is based on the use of multiple fuel jets in a single burner. Such burners may employ fuel staging, flue-gas recirculation, or a combination of both. U.S. Pat. Nos. 5,098,282 and 6,007,325 disclose burners using a combination of fuel-staging and flue-gas recirculation.
In the context of pre-mix burners, the term primary air refers to the air pre-mixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air are more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 4,629,413 discloses a low NOx pre-mix burner and discusses the advantages of pre-mix burners and methods to reduce NOx emissions. The pre-mix burner of U.S. Pat. No. 4,629,413 lowers NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The contents of U.S. Pat. No. 4,629,413 are incorporated by reference in their entirety.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from pre-mix burners by recirculating flue gas. Flue gas is drawn from the furnace through a pipe or pipes by the aspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. The flue gas mixes with combustion air in a primary air chamber prior to combustion to dilute the concentration of O2 in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing pre-mix burners or may be incorporated in new low NOx burners. The contents of U.S. Pat. No. 5,092,761 are incorporated by reference in their entirety.
Typical industrial furnaces for steam cracking or reforming employ multiple burners of the types described above. The burners described above typically are sized to fire from 0.3 to 2.5 MW (1-8 M Btu/hr). In contrast, even moderately sized industrial furnaces for reforming or steam cracking furnaces have a total fuel firing of from 30 to 150 MW. Accordingly such furnaces may have anywhere from 20 to over 100 burners.
Imbalance problems with flue gas recirculation and primary air exists when multiple burners are operated in a furnace. Due to the normal variations or tolerance in construction, leakage of air, partial fouling or plugging of components during operation or poor consistency in adjusting the burners there is considerable variability in FGR and primary air rates between individual burners in a furnace. In order to obtain the lowest NOx production in a furnace having multiple burners it is necessary to operate all the burners in the furnace at substantially similar FGR and primary air rates. This is particularly the case as more and more stringent requirements are adopted for NOx with respect to environmental considerations.
Furnaces of varied burner designs are used to reduce NOx emissions, and can benefit from the invention. Included are furnaces utilizing pre-mix burners with staged air to reduce NOx, furnaces with pre-mix burners and staged air and flue gas recirculation (FGR). Also included are furnaces utilizing pre-mix burners with staged fuel.
Despite these advances in the art, a need exists for an effective method for controlling the multiple burners used in an industrial furnace to meet the increasingly stringent NOx emission regulations, which minimizes localized sources of high NOx production.
Therefore, what is needed is a method to easily provide a means to adjust multiple burners in a furnace to minimize NOx production.